Present electronic equipment makes extensive use of on-board switching mode power supplies to change voltage levels from that supplied to the card to levels required by on-board circuitry. Measuring the current of on-board power supply switchers without modifying the circuit or de-soldering and lifting components from the PCB can be problematical.
There are a number of ways of measuring electrical current flow through a conductor. One such method consists of inserting a current measuring device, such as an ammeter, in series with the conductor; however, this requires breaking the conductor at the point of insertion. In many cases, such as with printed circuit boards, this breaking and inserting is difficult and inconvenient. Alternatively, removable components may be used, for example, a shorting link, which can be removed and then replaced, but this incurs the additional cost and space of such a component and its connector.
Preferably, the current measurement can be made non-invasively, for example, by detecting the magnetic field surrounding the conductor carrying the current. Referring to FIG. 1, there may be seen a switching mode power supply 100 having an output stage 110 with an output filter inductor 111. Conductive loop 112 connects the output stage 110 of filter inductor 111 with an output filter capacitor 114. Output voltage feedback line 116 connects the voltage on output filter capacitor 114 to regulation stage 118. Also seen is output load 120 which represents the load that switching mode power supply 100 is feeding. Magnetic probe 122 surrounds conductive loop 112 so as to sense the magnetic field produced by the output current. Magnetic probe 122 is connected to measuring instrument 124, which may be an averaging oscilloscope or other measurement device, which allows the magnitude of the current flowing through output filter inductor 111 to be ascertained. This approach has the disadvantage that inductance of conductive loop 112 and magnetic probe 122 changes the operational characteristics of the output circuit and may even cause loop instability if the applied inductance is comparable to a significant portion of the value of output filter inductor 111. As well, such a manual approach requires set up time and space which makes for measurement difficulties while the circuit card containing switching mode power supply 100 is in operation in an equipment shelf, or environmental test chambers. Additionally if measurements are going to be in the shelf and/or in the environmental chamber, size of the current probe is a limiting factor that may make it impossible to do the measurement.
A second possible solution uses a series resistor connected either in series with the output inductor or at the output of the switches in series with the load. Referring to FIG. 2, there may be seen a switching mode power supply 200 having an output stage 210 with an output filter inductor 211. Sense resistor 212 connects the output stage 210 of filter inductor 211 with an output filter capacitor 214. Output voltage feedback line 216 connects the voltage on output filter capacitor 214 to regulation stage 218. Also seen is output load 220 which represents the load that switching mode power supply 100 is feeding. Voltage sense leads 222 carry the voltage developed across sense resistor 212 to measurement unit 224. For high output current power supplies this approach is problematical at high currents due to the power dissipation developed within the sense resistor 212, as well as the addition to output impedance offered by the sense resistor 212. Further, the thermal losses in sense resistor 212 contribute to a loss in efficiency in the power supply 200. While possible implementations utilizing a plurality of resistors in parallel to cope with the power dissipation may be used, the real-estate cost issues on the circuit board are generally prohibitive.
A third possible approach is disclosed in an application note AN-7 “Loss-Less Current Sense Technique”, by LinFinity Microelectronics, incorporated by reference. This note describes a sense technique utilizing the parasitic resistance, RL, of the output filter inductor to sense the inductor current. A sensor consisting of a sense resistor RS and a sense capacitor CS are connected in series. The resultant assemblage is connected directly in parallel with the output filter inductor. The voltage VCs across sense capacitor CS is the sensor's output and is proportional to the output current of the inductor.
Referring to FIG. 3, there may be seen a switching mode power supply 300 having an output stage 310 with an output filter inductor 311. Sense resistor 312 and sense capacitor 313 are connected in series, and the resultant assemblage is connected across filter inductor 311. The output of filter inductor 311 is connected with output filter capacitor 314. Output voltage feedback line 316 connects the voltage on output filter capacitor 314 to regulation stage 318. Also seen is output load 320 which represents the load that switching mode power supply 300 is feeding. Voltage sense leads 322 carry the voltage developed across sense capacitor 313 to measurement unit 324.
A disadvantage of this solution is that it must be designed up front to be compatible with the output filter inductor 311. The values of sense resistor 312 and sense capacitor 313 are calculated as a function of the DC resistance value of output filter inductor 311. Typical manufacturer datasheets of the types of inductors used for output filter inductor 311 have wide tolerances which can result in measurement errors on the order of +1-10% due to variations in the nominal DC resistance. Further, as the windings of output inductor 311 will show a temperature dependency, further errors are introduced. Laboratory measurements have resulted in variations of up to 4% of error of the measured value per 10° C. As temperature rises of over 35° C. are not uncommon for this type of output inductor, it may be seen that considerable error can be introduced during operation. Attempts to compensate for the operating temperature are not simple, in that the effective operating temperature can be a function of power supply load, local ambient temperature, and surrounding cooling conditions. In general, this requires a complex method of measurement of output filter inductor's 311 temperature in order to compensate, and this works against a non-invasive measurement technique.
Therefore, it would be desirable to provide a circuit for non-invasively measuring current at the output of a switching mode power supply without the drawbacks described above and that would be applicable to existing circuit boards.